Han, 76, is the first former cabinet minister to be handed a ruling by a court on criminal charges directly related to martial law, which legal experts have said could prove a bellwether for other trials.
The Seoul Central District Court found Han guilty because he was instrumental in setting up the outward appearance of a cabinet meeting that facilitated the declaration, a judge said, a move he described as a "top-down insurrection".
Han also discussed plans to block the function of major institutions such as parliament as part of the insurrection, the judge added.
"The defendant was a prime minister who had been indirectly given democratic legitimacy and responsibility ... Nevertheless, the defendant chose to turn a blind eye ... and participate as a member of the December 3 insurrection," the judge said.
"As a result, South Korea was in danger of returning to the dark past when the basic rights and liberal democratic order of the people were violated, potentially preventing them from escaping ... dictatorship for a long time."
The court found Han guilty of "engaging in (the) key action of insurrection", as well as on related charges of perjury and forging an official document.
"I will humbly follow the judge's decision," Han, clad in a suit and green tie, said after the ruling on Wednesday.
He had denied wrongdoing on all charges except perjury, saying in November that while he regretted not being able to stop Yoon from declaring martial law, he "never agreed to it or tried to help".
In an unusual instance in South Korean court proceedings, the 23-year term exceeded a sentence of 15 years sought by prosecutors in November.
Han was detained by the court immediately after the ruling, which can be appealed. Han's lawyer said it would be taken to the Supreme Court.
Han is an experienced technocrat who served in senior posts under five presidents.
He became acting president after Yoon was impeached, before his own impeachment on accusations of having aided Yoon in the martial law declaration.
The Constitutional Court overturned Han's impeachment, restoring his powers to serve as leader before he resigned from the post to run in a snap election in June. He ended his bid for the presidency following rifts among conservatives.
Yoon, who faces eight separate trials, was handed a five-year jail term last week for charges that included obstructing attempts to arrest him following martial law. He has appealed.
Yoon faces another ruling on February 19 on the most serious charge of "masterminding an insurrection".
Prosecutors have sought the death penalty in that case, though South Korea has had no execution since 1997.
